In various electrical networks there are increasing opportunities to use load electronic systems to provide improved protection, automation and communication products for use with those electrical networks. It therefore becomes necessary to provide a suitable power supply for use by the load electronic systems.
In locations where a low voltage (LV) power supply is not available, or in applications where the electronic systems are preferably floated at high voltage (HV) line potential, there may be a prohibitive cost or space barriers in providing a power supply drawing its power from the HV line itself. The problem of providing power to load electronic systems is becoming more acute as the applications for load electronic systems expand.
As power requirements for load electronic systems are diminishing each year with the introduction of new semiconductor and communications technologies, an opportunity is provided for the provision of limited power from the line voltage of HV power supplies at low cost.
Voltage power supplies utilizing HV electrical networks have to overcome several problems.
One such problem is that HV electrical networks need to withstand high electrical stresses due to the high system voltage being applied. These therefore require appropriate insulation systems with due regard to surface tracking, material breakdown, partial discharge and so on. There are standard production tests that may be used to monitor electrical stress, such as, for example, power frequency (PF) tests and partial discharge tests.
Further, these electrical networks need to withstand high over-voltage impulses which may occur in electrical networks. These are usually caused by lightning or connected switching devices and can be 100 kV or more on electrical networks. There are standard design tests that may be used to monitor for this condition, such as, for example, lightning impulse tests etc.
Also, the consequences of insulation failure are usually catastrophic and can result in explosions because of the high voltages and high energies involved. This can lead to a significant design effort in order to limit the consequences of failure. Also, this can result in over-design, which can further result in increased product cost.
Overcoming these problems can therefore lead to large, heavy and expensive solutions with high installation costs to the utility.
It is known to derive power from the line connection on a HV network to operate remote equipment, such as a recloser for example, where the power is obtained via a wound voltage transformer (VT). This has been considered necessary for the high power requirement of the controllers (approximately 20-50 W) which operate at ground potential.
However, design and construction of a wound VT is complex and costly. Also, reducing the power requirements for the wound VT does not reduce the cost in proportion. For example, a 200 mW VT is not one thousandth of the cost of a 200 W VT.
A wound VT is a well-established solution that comes at high installation and purchase installation cost. Moreover for certain products, such as the Fusesaver product offered by Siemens, or indeed other equipment running at line potential, the requirement would be for multiple VT's or a special purpose design with multiple isolated secondary windings in order to supply the electronics that are at line potential on each phase. This therefore increases costs further.
Although the wound VT may be a good solution for applications requiring several watts of power, it is not considered a good solution for use with applications requiring less than one watt of power. These may include, for example, applications such as capacitor bank switches and reclosers operating at line potential, as well as fault indicators and line quality monitors etc.
High voltage ceramic capacitors have been used previously for low power controllers. According to this method, the HV line is fed through the capacitor to a grounded power transformer (VT) with a primary voltage that is much lower than the line voltage. In theory, the cost of lower voltage transformer is much lower than that of a high voltage transformer.
Special purpose capacitors have also been developed for high voltage electrical systems for this purpose. Although they are cheaper than a wound VT, they still require insulation systems to be engineered. Further, these special purpose capacitors may suffer from significant problems. For example, these special purpose capacitors may not withstand lightning impulse voltages of the required magnitude. Further, on impulse they may offer low impedance to the wave front and hence apply a high or very high voltage surge to the transformer which means the transformer design is complex and/or additional protection components must be incorporated.
These fundamental problems make use of these special purpose capacitors in conjunction with a transformer very problematic and so are not considered a viable solution to the problem.
It may also be possible to use an inductor in series with a transformer to limit the current at line voltage. This has the advantage of limiting the voltage applied to the transformer during impulse, as the impulse voltage is withstood across the inductor. However, the design and construction of the inductor is almost as complex as the design for a VT, where they have to solve the problem of withstanding full impulse voltage across a wound inductor.
Therefore, series inductors are not considered a viable solution to the problem.
Another possible solution would be to use a resistor in series with the transformer as this is much easier to design from the perspective of withstanding a high impulse voltage.
However, the resistor power dissipation will usually be high at normal line voltages. For example, for a 22 kV line with a 1 mA resistor current, a dissipation of 13 W is required which can result in significant heating.
When performing a PF test, power dissipation may increase further. For example, 13 W power dissipation may become 200 W during a 50 kV PF test, which may be impossible to manage. Furthermore, if a higher primary current is required (e.g. 2 mA) then these power dissipations may double again.
Therefore, a series resistor in conjunction with a transformer is not considered a viable solution to the problem.